Hydrogen sulfide abating cooling toner

ABSTRACT

Hydrogen sulfide is removed from a fluid stream of condensed steam by addition of an iron chelate catalyst to oxidize H 2  S to sulfur. The removal is carried out in a modified cooling tower where condensed steam is added to a recycled iron chelate liquid in a water collection tray for a period of time sufficient to oxidize the H 2  S before the fluid stream is introduced to the cooling tower to remove sulfur.

DESCRIPTION

1. Technical Field

The invention relates to the removal of hydrogen sulfide from a fluidstream which contains water, usually condensed steam, in addition to thehydrogen sulfide. The removal is carried out utilizing a modifiedcooling tower of the nature used for cooling and evaporating geothermalwaste steam.

2. Background of the Invention

Sour water, which constitutes water plus hydrogen sulfide and, at times,other impurities, is produced in a number of processes. For example, theoutput steam and hydrogen sulfide mixture from a turbine run ongeothermal steam is conventionally condensed to form sour water. Sourwater can also be formed in various industrial processes such as thehydrotreating of gasoline and fuel oil followed by washing of the gasesthereby produced. Sour water can similarly be produced by hydrotreatingof coal.

In the geothermal steam industry it is customary to condense the outputof a turbine driven by the geothermal steam in a condenser or series ofcondensers, to combine a catalyst, often ferric chelate, with thecondensate output of the condensers in what is generally known as a hotwell and is generally at a temperature of between about 100° and 125° F.and then to deliver the water from the hot well to the top of a coolingtower, perhaps 50 feet in height, through which it flows downwardlyagainst the upward flow of a stream of fast moving air. In the coolingtower approximately 80% of the water volume of the condensed steam isevaporated and any residual hydrogen sulfide is desorbed into theatmosphere. The remaining water, comprising approximately 20% of thevolume of the condensed steam plus the volume of cooling water providedto the condenser or condensers, reaches the bottom of the tower where itflows into a tray. The overflow from the tray is discarded while part ofthe liquid from the tray is recycled as cooling liquid to the condenseror condensers. Such systems are described in, for example, U.S. Pat. No.4,414,817, issued Nov. 15, 1983 to R.T. Jernigan, U.S. Pat. No. 4,528,817, issued July 16, 1985 to R.T. Jernigan, U.S. Pat. No. 4,614,644,issued Sept. 30, 1986 to R.D. Lampton, Jr., and T.M. Hopkins II, and inU.S. Pat. No. 4,363,215, issued Dec. 14, 1982 to S.G. Sharp.

As has been noted above a portion of the cool water from the tray at thebottom of the cooling tower is discarded in liquid form. In essence, 80%of the water condensed from the turbine exhaust is evaporated in thetower while the remaining 20% must constitute the overflow for thesystem to operate in equilibrium. The overflow water carries with it asignificant amount of the catalyst which was present in the hot wellthereby leading to very significant processing costs. And, theconcentration of ferric chelate or other catalyst, e.g., a polyvalentcobalt, manganese, tin, vanadium, platinum, palladium molybdenum,chromium, copper or nickel chelate, in the hot well must be relativelyhigh to assure reasonably complete conversion of hydrogen sulfide tosulfur because the time of retention of the condensed sour water in thehot well and associated piping connecting it to the top of the coolingtower is very short, of the order of 30 seconds or less, and usually inthe range of from about 5 to about 15 seconds. The kinetics of thereactions whereby the hydrogen sulfide is oxidized with the concomitantreduction of the ferric chelate to ferrous chelate and the catalyst isregenerated to the ferric state are such that if the oxidation reactionis to go reasonably towards completion it is necessary, as previouslystated, that a relatively high concentration of ferric chelate catalystbe maintained.

The present invention is directed to overcoming one or more of theproblems as set forth above.

DISCLOSURE OF INVENTION

In accordance with an embodiment of the present invention an improvementis set forth in a process for removing H₂ S from a condensate comprisingH₂ S and water which comprises including with the condensate a catalystin an amount sufficient to catalyze the oxidation of the H₂ S to sulfur,introducing the condensate to the top portion of the cooling tower fromwhich it flows downwardly against an upward flow of air whereby it iscooled and oxygenated and a portion of the condensate is evaporated toform a recycle liquid, collecting the recycle liquid in a watercollection tray, discarding a portion of the recycle liquidcorresponding generally to the non-evaporated portion of the condensateand recycling the remainder of the catalyst containing recycle liquidback to the condenser. The improvement comprises carrying out theintroducing step by adding the condensate to the recycle liquid in thewater collection tray at a first region thereof to form a recycleliquid-condensate mixture. A portion of the mixture is removed from asecond region of the water collection tray. The second region is spacedapart from the first region a selected distance. The thus removedportion of the mixture is introduced to the top of the cooling towerwhereby the condensate is thereby introduced to the top of the toweralong with a proportionate amount of the recycle liquid.

In accordance with another embodiment of the present invention animprovement is set forth in a process for removing H₂ S from a fluidstream having steam as its major component which comprises condensingthe steam to form a condensate, including a catalyst with the condensatein an amount sufficient to catalyze the oxidation of the H₂ S to sulfur,introducing the condensate to the top portion of a cooling tower fromwhich it flows downwardly against an upward flow of air whereby it iscooled and oxygenated and a portion of the condensate is evaporated toform a recycle liquid, collecting the recycle liquid in a watercollection tray, discarding a portion of the recycle liquidcorresponding generally in volume to the non-evaporated portion of thecondensate and recycling the remainder of the catalyst containingrecycle liquid back to the condenser. The improvement comprises carryingout the introducing step by adding the condensate to the recycle liquidin the water collection tray at a first region thereof to form a recycleliquid-condensate mixture A portion of the mixture is removed from asecond region of the water collection tray. The second region is spacedapart from the first region a selected difference. The thus removedportion of the mixture is introduced to the top of the cooling towerwhereby the condensate is introduced to the top of the cooling toweralong with a proportionate amount of the recycle liquid.

The present invention allows use of much lower quantities, generally nomore than about one half the amount, of catalyst, generally ironchelate, to equally well remove a given quantity of H₂ S from a fluidstream. This is accomplished by, in essence, greatly increasing thereaction time available for the oxidation of the H₂ S to sulfur and byincreasing the available oxygen in solution to bring this about. Thistime is greatly increased by taking the condensate and introducing it tothe tray at the bottom of the cooling tower and then removing themixture of the recycle liquid in the tray plus the added condensate froma different location in the tray and introducing that mixture to the topof the cooling tower. The added reaction time is the time it takes thecondensate to flow from the region where it is introduced into thecooling tower collecting tray to the region from which it is abstracted.Cooling tower water collecting trays are far bigger than the hot wellsof condensers (a typical hot well and associated piping might have avolume of 10,000 gallons while a typical cooling tower collecting traymight have a volume of 250,000 gallons). Much longer residence times areavailable and possible in the cooling tray than are available andpossible in the hot well since approximately the same rate of liquidflow occurs through both. Thus, time in the cooling tray will generallybe at least one minute, and can easily be made to be several minutes inlength. The result is the use of far less catalyst, generally ironchelate, in the process with very significant monetary savings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the figures ofthe drawings wherein like numerals denote like parts throughout andwherein:

FIG. 1 illustrates a process for purifying geothermal steam inaccordance with the present invention; and

FIG. 2 illustrates the portion of the process taking place in thecooling tower collecting tray.

BEST MODE FOR CARRYING OUT INVENTION

FIG. 1 illustrates a process in which the invention is applied for theoxidation of hydrogen sulfide contained in a liquid stream produced bythe condensation of geothermal steam. A similar application can be madeto streams resulting from other industrial processes.

In FIG. 1 the geothermal steam from line 2 is used to power a steamturbine 4 which is connected to an electric power generator 6. Theturbine 4 exhausts through line 8 to a direct contact condenser 10.Cooling water containing chelated iron (ferric chelate), or otherappropriate catalyst, from line 12 is sprayed into condenser 10 for thiscondensation and passes from the condenser 10 through line 14 to the hotwell 16 operating at, usually, 100°-125° F. Non-condensible gases suchas CO₂, H₂, CH₄, N₂ and H₂ S, along with atmospheric gases which canleak into the condenser 10, are removed from the main condenser 10through the line 18 by two steam jet ejectors 20 and 34 and theassociated condensers 26 and 42. The ejectors 20 and 34 are operated bysteam supplied by lines 22 and 36, respectively. These ejectors create apartial vacuum or low pressure zone. The exhaust flow stream from theejector 20 is carried by line 24 to the condenser 26 and by line 32 tothe second ejector 34. The exhaust steam from ejector 34 is carried byline 40 to condenser 42. Cooling water from line 12 is supplied to eachof the condensers 26 and 42 by lines 28 and 44, respectively. Thecondensed steam from the condensers 26 and 42 flows by means of line 30and 46 to the hot well 16. The hydrogen sulfide component of thenon-condensible gases can be scrubbed by addition of caustic to thequench water supply to the condenser 42 or the condenser off-gas ventedto a conventional incinerator/scrubber unit through line 48 for removalof the H₂ S gases.

Pump 58 is used to pump the combined thermal steam and cooling waterfrom hot well 16 through line 60, and in accordance with the invention,to a first region 61 in collecting tray 63 located at the base of thecooling tower 62. Also in accordance with the invention a portion of themixture of condensate and recycle liquid in the collecting tray 63 ispumped from a second region 65 in the collecting tray 63, the secondregion 65 being spaced apart a selected distance from the first region61, via a conduit 67 and under the impetus of a pump 69 to internalspray heads 66 in the top portion of the cooling tower 62. An amount ofwater equal to approximately 80% of the condensed steam is evaporated byair flow through the tower which also strips all of the dissolvedhydrogen sulfide from the liquid whereby it would be vented to theenvironment by means of the air stream 64 if it had not already beenconverted to sulfur by use of the chelated iron. The excess condensedsteam which is not evaporated overflows the collection tray 63 fordisposal by line 80. The remainder of the cooled recycleliquid-condensate mixture flows through line 68 to the relatively coldwell 70 which operates at 75°-85° F. Pump 50 is used to pump the recycleliquid-condensate mixture from the cold well 70 to the condensers 42, 26and 10. In the embodiment illustrated the hot well 16 is separated fromthe cold well 70 by a weir 72.

In order to prevent the release of the dissolved hydrogen sulfide to theenvironment in the air stream 64 flowing from the top of the coolingtower 62, a catalyst, often ferric chelate, is added to the circulatingwater in an amount sufficient to catalyze the oxidation of most of thedissolved hydrogen sulfide in the hot well 16 and in the collecting tray63. The iron chelate is concurrently oxidized from the ferrous to theferric state by any dissolved oxygen. In this manner, the dissolvedhydrogen sulfide is oxidized before the water enters the top of thecooling tower 62 from the conduit 67. The air flow and time of contactbetween the air and water in the cooling tower 62 is sufficiently longso that the ferrous chelate, which results from the oxidation of thedissolved hydrogen sulfide in the hot well 16 and in the collecting tray63 and also in associated piping 56, 60 and 67, is reoxidized fully tothe active ferric state as it passes down through the cooling tower 62.Concurrently, the water is oxygenated. Elemental sulfur produced by theprocess may be eliminated by conventional means from the overflow line80.

In order to maintain at least the minimum amount of iron chelaterequired for this process, an amount of fresh iron chelate equal to theamount lost in the overflow line 80 and that amount of iron lost toformation of insoluble iron compounds in the solution is added from thestorage vessel 74 by pump 76 and inlet line 78. It should be noted thatthe point of addition of the additional iron chelate to the circulatingwater is a matter of design choice. That is, it is not necessary that itbe added at the particular location shown. Instead, it can be added, forexample, to the hot well 16, the cold well 70 or the collecting tray 63.Thus, the ferric chelate may be included with the condensate by addingit substantially anywhere in the water flow cycle. To minimize loss ofiron to overflow and loss by solids formation (especially when causticscrubbing occurs in the after condenser 42) it works well to add theiron chelate where illustrated in FIG. 1.

In accordance with the present invention the selected distance betweenthe first region 61 and the second region 65 in the collecting tray 63is selected to provide sufficient residence time of the recycleliquid-condensate mixture in the water collection tray 63 so that theoxidation of the H₂ S is carried out substantially to completion. Thisreaction is facilitated by the oxygen rich water which falls through thecooling tower 62. Generally the residence time will be selected to be atleast about one minute. The overall effect of utilizing a process as setforth herein is that the concentration of ferric chelate which isincluded in the condensate is reduced by at least a factor of two andsuitably by a factor of five or more. The current cost for iron chelateand associated H₂ S abatement chemicals for the production of about 350megawatts at a portion of the geyser fields in the Geyserville area ofNorthern California is of the order of Ten Million Dollars per year.Thus, a very significant saving results from operation in accordancewith the present invention.

Referring principally to FIG. 2 an embodiment of the invention isillustrated wherein flow through the cooling tray 63 is controlled sothat recycle liquid-condensate mixture which is extracted from thesecond region 65 of the tray 63 is of a somewhat more uniformcomposition. Flow of the mixture takes place from the first region 61 ofthe tray 63, where the condensate is introduced to the collecting tray63, rightwardly as directed by a plurality of walls 82 till the flowfinally passes out via the line 80. The second region 65 can be solocated as to provide water of a desired temperature for recycling viathe line 67 to the top portion of a plurality of cooling towers 62, allfeeding into the same collecting tray 63. It should be noted that sincethe recycle liquid-condensate mixture is at a somewhat coolertemperature than the condensate alone, the temperature Of the mixtureintroduced to the top portion of the cooling tower 62 is somewhatlowered over that which was introduced by the prior art. The temperatureof the cooling water in the cooling tower tray 63 will also be somewhatwarmer at the outlet 80. Both depend on the proximity of line 60 toconduit 67. Accordingly, somewhat less water will be evaporated in thecooling tower 62. If desired, the cooling tower 62 can be increased insize whereby just as much water can be evaporated as is currentlyevaporated.

The terms ferric chelate, ferrous chelate and iron chelate are used inthe usual sense with respect to the removal of H₂ S from sour gases andsour water. A conventional removal process which utiIizes iron chelateis shown in for example U.S. Pat. Nos. 4,091,073 and 4,076,621.Basically the present invention contemplates the use of any chelatediron solution or other suitable polyvalent metal catalyst having both ahigher and a lower oxidation state and which is operable for removinghydrogen sulfide. It is also possible to use two different types ofchelating agents, one of which is selected to bind ferrous ions stronglyenough to prevent precipitation of ferrous sulfide and the other ofwhich is selected to bind ferric ions strongly enough to preventprecipitation of ferric hydroxide. Such chelating agents are availablecommercially for use in converting H₂ S to sulfur.

Industrial Applicability

The present invention provides processes for removing H₂ S from a fluidstream having steam as its major component, which fluid stream may beobtained from a steam turbine which is powered by steam from ageothermal well. The amount of chelated iron needed to participate inthe oxidation-reduction reaction which cyclically produces sulfur fromhydrogen sulfide is significantly reduced when operating in accordancewith the present invention. As a result, the total amount of ironchelate utilized in the process is significantly less than that utilizedin prior art processes for removing H₂ S from a fluid stream havingsteam as its major component.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention and the limits of the appended claims.

That which is claimed is:
 1. In a process for removing H₂ S from acondensate comprising H₂ S and water which comprises including with thecondensate a catalyst in an amount sufficient to oxidize the H₂ S tosulfur, introducing the condensate to the top portion of a cooling towerfrom which it flows downwardly against an upward flow of air whereby itis cooled and oxygenated and a portion of the condensate is evaporatedto form a recycle liquid, collecting the recycle liquid in a watercollection tray, discarding a portion of the recycle liquidcorresponding generally to the non-evaporated portion of the condensateand recycling the remainder of the catalyst containing recycle liquidback to the condensate, an improvement comprising, carrying out saidintroducing step by:adding the condensate to the recycle liquid in thewater collection tray at first region thereof to form a recycleliquid-condensate mixture; removing a portion of the mixture from asecond region of the water collection try, said second region beingspaced apart from said first region a distance selected to providesufficient residence time of the mixture in the water collection try forthe oxidation of the H₂ S to be carried substantially to completion; andintroducing the thus removed portion of the mixture to the top of thecooling tower whereby the condensate is thereby introduced to the top ofthe tower along with a proportionate amount of the recycle liquid.
 2. Aprocess as set forth in claim 1, wherein said residence time is at leastabout 1 minute.
 3. A process as set forth in claim 1, wherein the amountof catalyst which is included in the condensate is reduced by at least afactor of 2 from that amount which is needed for the oxidation of the H₂S to be substantially completed if the condensate is introduced directlyto the top portion of the cooling tower.
 4. A process as set forth inclaim 1, wherein the catalyst has a higher oxidation state and a loweroxidation state, wherein the catalyst is in its higher oxidation statewhen included with the condensate, is reduced to its lower oxidationstate during oxidation of the H₂ S to sulfur while concurrently beingoxidized to its higher oxidation state by oxygen dissolved in thecondensate and is oxidized to its higher oxidation state as thecondensate flows downwardly through the cooling tower.
 5. In a processfor removing H₂ S from a fluid stream having steam as its majorcomponent which comprises condensing the steam to form a condensate,including a catalyst with the condensate in an amount sufficient tooxidize the H₂ S to sulfur, introducing the condensate to the topportion of a cooling tower from which it flows downwardly against anupward flow of air whereby it is cooled and oxygenated and a portion ofthe condensate is evaporated to form a recycle liquid, collecting therecycle liquid in a water collection tray, discarding a portion of therecycle liquid corresponding generally to the non-evaporated portion ofthe condensate and recycling the remainder of the catalyst containingrecycle liquid back to the condensate, an improvement comprising,carrying out said introducing step by:adding the condensate to therecycle liquid in the water collection tray at a first region thereof toform a recycle liquid-condensate mixture; removing a portion of themixture from a second region of the water collection tray, said secondregion being spaced apart from said first region a distance selected toprovide sufficient residence time of the mixture in the water collectiontry for the oxidation of the H₂ S to be carried substantially tocompletion; and introducing the thus removed portion of the mixture tothe top of the cooling tower whereby the condensate is therebyintroduced to the top of the tower along with a proportionate amount ofthe recycle liquid.
 6. A process as set forth in claim 5, wherein saidresidence time is at least about 1 minute.
 7. A process as set forth inclaim 5, wherein the amount of catalyst which is concluded in thecondensate is reduced by at least a factor of 2 from that amount whichis needed for the oxidation of the H₂ S to be substantially completed ifthe condensate is introduced directly to the top portion of the coolingtower.
 8. A process as set forth in claim 5, wherein the catalyst has ahigher oxidation state and a lower oxidation state, wherein the catalystis in its higher oxidation state when included with the condensate, isreduced to its lower oxidation state during oxidation of the H₂ S tosulfur while concurrently being oxidized to its higher oxidation stateby oxygen dissolved in the condensate and is oxidized to its higheroxidation state as the condensate flows downwardly through the coolingtower.